python - Non blocking read on os.pipe on Windows

Question

This question - How to read from an os.pipe() without getting blocked? - shows a solution how to check if os.pipe has any data for Linux, and for this you need to put the pipe into non-blocking mode:

import os, fcntl
fcntl.fcntl(thePipe, fcntl.F_SETFL, os.O_NONBLOCK)

On Windows we have this:

ImportError: No module named fcntl

But os.pipe is there:

>>> os.pipe()
(3, 4)

So, is it possible to do non-blocking read or peek the contents of os.pipe on Windows?

Answer 1

Answering my own question after digging for some time through StackOverflow.

UPDATE: Things changes thanks to @HarryJohnston.

At first the answer was no, it is not possible to do non-blocking read on os.pipe on Windows. From this answer I've got that:

The term for non-blocking / asynchronous I/O in Windows is 'overlapped' - that's what you should be looking at.

os.pipe on Windows is implemented through CreatePipe API (see here and ... well, I couldn't find os.pipe code in Python sources). CreatePipe makes anonymous pipes, and anonymous pipes do not support asynchronous I/O.

But then @HarryJohnston commented that SetNamedPipeHandleState doc allows to put anonymous pipe to non-blocking mode. I wrote the test and it failed with OSError: [Errno 22] Invalid argument. The error message seemed wrong, so I tried to check what should be return result on non-blocking read operation when data is not available, and after reading MSDN note on named pipe modes I found that it should be ERROR_NO_DATA that has a int value 232. Adding ctypes.WinError() call to exception handler revealed the expected [Error 232] The pipe is being closed.

So, the answer is yes, it is possible to do non-blocking read on os.pipe on Windows, and here is the proof:

import msvcrt
import os

from ctypes import windll, byref, wintypes, GetLastError, WinError
from ctypes.wintypes import HANDLE, DWORD, POINTER, BOOL

LPDWORD = POINTER(DWORD)

PIPE_NOWAIT = wintypes.DWORD(0x00000001)

ERROR_NO_DATA = 232

def pipe_no_wait(pipefd):
  """ pipefd is a integer as returned by os.pipe """

  SetNamedPipeHandleState = windll.kernel32.SetNamedPipeHandleState
  SetNamedPipeHandleState.argtypes = [HANDLE, LPDWORD, LPDWORD, LPDWORD]
  SetNamedPipeHandleState.restype = BOOL

  h = msvcrt.get_osfhandle(pipefd)

  res = windll.kernel32.SetNamedPipeHandleState(h, byref(PIPE_NOWAIT), None, None)
  if res == 0:
      print(WinError())
      return False
  return True


if __name__  == '__main__':
  # CreatePipe
  r, w = os.pipe()

  pipe_no_wait(r)

  print os.write(w, 'xxx')
  print os.read(r, 1024)
  try:
    print os.write(w, 'yyy')
    print os.read(r, 1024)
    print os.read(r, 1024)
  except OSError as e:
    print dir(e), e.errno, GetLastError()
    print(WinError())
    if GetLastError() != ERROR_NO_DATA:
        raise